1. Field of the Invention
The present invention a luminescent storage screen having a stimulable phosphor for the latent storage of x-ray images, of the type wherein the x-ray image is read-out by exciting the phosphor with a read-out radiation beam.
2. Description of the Prior Art and Related Applications
Luminescent storage screens for the latent storage of x-ray images are known in the art which contain a stimulable phosphor, the stored x-ray image being read-out by exciting the phosphor with a read-out radiation beam of a first wavelength, causing radiation of a second wavelength to be emitted by the phosphor, which is acquired by a detector unit, as described, for example, in European Application 0 369 049.
Such luminescent storage screens are used in image pick-up devices as disclosed, for example, in German OS 23 63 995. These types of storage screens function as a radiation-sensitive transducer in x-ray diagnostics installations. When the screen is irradiated with x-rays, electronic holes are generated in the stimulable phosphor in accordance with the intensity of the incident radiation. These holes are stored in energy traps having a higher energy level, so that a latent x-ray image is contained in the luminescent storage screen.
For read-out of the latent image, the entire surface of the luminescent storage screen is caused to luminesce pixel-by-pixel by a separate radiation source which may be, for example, a laser. This source generates stimulating radiation at a first wavelength, which raises the energy level of the holes stored in the traps, so that they can degenerate to lower energy levels, the energy difference being emitted in the form of light quanta. As a result, the stimulable phosphor emits light at a second wavelength dependent on the energy stored in the stimulable phosphor. The light emitted due to the stimulation is detected and made visible, so that the latently stored x-ray image can be visually displayed.
A problem in the read-out of such known storage screens is that the stimulable phosphor is not sufficiently transparent for the laser light. A minimum thickness of the stimulable phosphor is required in order to achieve adequate absorptions of x-ray quanta. In the case of a non-transparent, densely compressed or sintered phosphor, the laser beam is so highly attenuated by the phosphor during read-out that the penetration depth of the laser beam is too small. After a certain depth within the phosphor, the energy of the laser beam is no longer sufficient to boost the holes to the energy level required for the recombination, so that the information stored in the deeper layers of the phosphor cannot be read out.
A luminescent storage screen is disclosed in European Application 0 369 049, corresponding to co-pending U.S. application Ser. No. 653,950 (Brandner et al., filed Feb. 12, 1991) which is a continuation of Ser. No. 419,784 (filed Oct. 11, 1989, now abandoned), wherein a stimulable phosphor is vapor-deposited onto a carrier in a high vacuum and is tempered in a protective gas atmosphere or in the vacuum, or is pressed in a vacuum while being heated. The production of a transparent phosphor is disclosed in European Application 90102431.5, corresponding to co-pending U.S. application Ser. No. 643,506 (Brandner et al., filed Jan. 22, 1991), by re-shaping transparent stimulable phosphor single crystals to the large area required for medical diagnostics by pressing. This results in the production of a transparent panel of stimulable phosphor. The advantage of transparency is that the laser beam used for read-out is not dispersed in the storage medium due to scattering at the grains of the phosphor material. The broadening of dispersal of the read-out beam due to scatter considerably deteriorates the modulation transfer function of the overall system. The broadening or dispersal of the laser beam upon transirradiation of the storage medium is substantially diminished by the use of a transparent stimulable phosphor which is manufactured, for example, by pressing the phosphor powder.
The problem of direct reflection at the boundary surfaces of the stimulable phosphor is present to a far greater degree in the case of transparent phosphors than in the case of non-transparent phosphor layers which have diffuse reflections. This problem is explained in greater detail with reference to FIG. 1. For pixel-by-pixel read-out of the x-ray image, the exciting beam, having a first wavelength, penetrates the luminescent storage screen 1 which, for example, may consist of a carrier and a binding agent applied thereon with the stimulable phosphor, or may consist of a single-crystal stimulable phosphor. In any event, the beam 2 is incident on the stimulable phosphor 3 which, as a result of such excitation, emits rays 4 at a second wavelength with a spherically symmetrical distribution. Radiation is thus emitted at all angles relative to the boundary surface.
Because, however, the refractive index n of the stimulable phosphor is higher in all cases than that of air or a vacuum (n'-1), a total reflection occurs starting with a defined incident angle of the luminescent light relative to the boundary surface, as set forth in detail in FIG. 2. As a result, only a portion of the light can emerge from the desired exit face.
Given total reflection, the boundary angle e is generally calculated based on the relationship e=arcsin n'/n. The solid angle at which exit of the beam occurs is R=2.pi.(1-cos e). For the transparent stimulable phosphor RbBr having a refractive index n=1.55, a boundary angle of 40.18.degree. is obtained for the total reflection, with the solid angle than being 1.48225 sr, which constitutes only 11.8% of the full volume 4.pi.. Only 11.8% of the luminescent light thus emerges from the desired exit face. If the face lying opposite to the desired exit face is provided with a coating which functions as a mirror in the wavelength range of the luminescent light, then this portion of the light which would emerge through this face can be reflected to the desired exit face. The portion of the light which is sought can thus be doubled in the ideal case. Nonetheless, even in this ideal case only 23.6% of the total light can be obtained.
When the lateral faces of the luminescent storage screen are disposed perpendicularly relative to the end faces (the end faces being the face through which radiation enters the storage screen, and the face parallel thereto), the same light portion emerges through the lateral faces, because all light rays which were totally reflected at the end face will be incident on the lateral faces at an angle of 90.degree.-e. This is illustrated by the geometrical conditions illustrated in FIG. 3. A first ray a of the entering rays 4 is incident on a first end face 5 at an angle of .alpha..sub.1 =45.degree. and is totally reflected because the angle is larger than the boundary angle e=40.18.degree.. The reflected ray a' is incident on one of the lateral faces 6 at an angle .alpha..sub.2 =.alpha..sub.1 =45.degree., so that it is also reflected at that face.
If, as in the case of the ray b, the angle is greater than approximately 50.degree., the ray b is incident on the end face 5 at an angle .beta..sub.1 and is totally reflected at that face. The reflected ray is incident on the lateral face at an angle of incidence .beta..sub.2, which is less than 40.degree., so that the ray b' can emerge refracted from the storage luminescent screen 1.
Only for completeness, a ray c is also shown which is incident on the end face 5 at an angle of incidence .gamma..sub.1 =30.degree.&lt;e=40.12.degree., which emerges from this end face 5 at a refracting angle .gamma..sub.2 =50.8.degree..
As is apparent from these explanations, a portion of the light emitted in the luminescent storage screen 1 cannot emerge from the screen due to total reflections.